The present invention relates to an electrochemical secondary cell having at least one positive electrode with an active material comprised of a lithium-intercalating chalcogen compound of a transition metal, at least one negative electrode with an active material comprised of a lithium-intercalating carbon product, and a non-aqueous electrolyte in a sealed container.
As is well known, the use of lithium metal electrodes in rechargeable cells is subject to severe constraints, due to the tendency of the lithium toward dendrite formation and shedding. However, an extraordinary improvement in the cycling of lithium cells has been made possible with electron-conductive matrix substances which, in the charging/discharging mode, can alternately be loaded with lithium ions as the electrochemically active ion species, and thereafter similarly depleted of such ions (see, e.g., U.S. Pat. No. 4,828,834).
Alluding to the "swing rhythm" by which the lithium alternates between the host lattice of the electrode of one polarity and the host lattice of the electrode of the other polarity, these novel reversible battery systems are sometimes referred to as "SWING systems" by those skilled in the art. In the technical literature, the designations "rocking chair cells" and "lithium ion cells" are also found.
Lithium secondary cells operating as SWING cells usually have, as the lithium-intercalating support matrix substance of the positive electrode, a lithium manganese spinel, LiMn.sub.2 O.sub.4, or a lithiated transition metal oxide such as LiCoO.sub.2 or LiNiO.sub.2. In the manganese spinel, some of the manganese may be substituted by other transition metals (e.g., Co and Ni) for the purpose of stabilizing the spinel lattice. The matrix substance of the negative electrode is generally needle coke, an irregularly crystallized carbon product generated by a slow coking process from an organic material, or graphite.
Electrodes for SWING cells containing liquid electrolytes have reached a high degree of technical sophistication, because they are fabricated by processes which parallel other sectors of the industry. For example, in accordance with European Patent No. 205,856, thin-film electrodes having an overall thickness of only 100 .mu.m can be fabricated by matrix substances of the above-mentioned type in finely powdered form (mixed with a conducting medium, if required), pasted with a solution of an adhesive in an organic solvent. The obtained mass is spread on foils made of aluminum or copper (by means of a knife).
The cell types which most suitably use such thin-film electrodes are, as expected, the round cell and the wound cell. However, an unfavorable economy of space and associated heat dissipation problems (which increase with size) render such cells inferior to prismatic cells (which are becoming of greater interest in certain fields of application for batteries, primarily for electrically powered vehicles).
On the other hand, providing prismatic cell containers with known thin-film electrodes tends not to provide a remedy since many thin-film electrodes (in the form of a relatively large, sandwich-like pack) require a high degree of separation, to the detriment of energy density, and are not easily handled.
Thick, knife-coated electrodes having an overall thickness of more than 200 .mu.m cannot be implemented because, with their greater layer thicknesses, adhesion problems tend to occur on the substrate foil. Moreover, an increase in energy density by lowering the porosity (&lt;30%) cannot be achieved in the case of knife-coated electrodes, because excessive rolling leads to their deformation.